The Elder Scrolls IV: Oblivion - Mage Guild Reinstatement Mini-FAQ

Mages' Guild Reinstatement: Finding Redwort Flowers or Dragon's Tongue
Try alchemy shops and Mage Guild NPC's if you want to buy them, although
you'll probably have to wait for enough quantity to respawn.
If you want to go flower picking according to the Strategy Guide a large
concentration of Dragon's Tongue flowers can be found around Fort Redwater
near the Black Marsh border, some NE of Leyawiin, and a few found in the West
Weald region between Skingrad and Bravil.
Redwort flowers are SE of Fieldhouse Cave in Leyawiin County near Black Marsh
border, and around the Aeylid ruins of Wendelbek and Morahame along the
Panther river on the east side of Nibena Bay. There are also small amounts
found in the West Weald region between Skingrad and Bravil.
NOTE: Several people have posted that all of the faction 'penance' quests
where you are told to get X amount of 2 different items is actually either/or
instead of both despite what you are told.
In other words you may only need 20 Dragon's Tongue or 20 Redwort Flowers, not
both to get back in.